The present invention relates to a power converter with an insulated gate semiconductor device of a MOS gate structure, for example MOS-FET, IGBT (Insulated Gate Bipolar Transistor), IEGT (Injection Enhanced Gate Transistor) as a voltage drive switching element.
There is available a voltage drive switching element as a switching element used in a power converter, for example an inverter which drives an induction motor converting a direct current power to an alternating current power other than a current drive switching element such as a thyristor, a gate turn-off thyristor (GTO) or transistor. As typical examples of a voltage drive switching element, there are named: an insulated gate semiconductor device of a MOS gate structure, for example MOS-FET, IGBT (Insulated Gate Bipolar Transistor), IEGT (Injection Enhanced Gate Transistor).
FIG. 1 shows a gate drive circuit of IGBT in a conventional power converter. In FIG. 1, if a switching command v.sub.in for placing IGBT 7 in the conductive state (on) or non-conductive state (off), a gate voltage v.sub.ge corresponding to the switching command v.sub.in is applied between the gate and emitter of the IGBT 7 through transistors 3, 4 and a gate resistor 6. Thereby a switching operation such as turn-on or turn-off is performed in the IGBT 7. That is, if a positive switching command v.sub.in is input from a gate control circuit not shown as shown in FIG. 2, the transistor 3 is placed in the on state and the transistor 4 is placed in the off state, the output voltage v.sub.g becomes a positive voltage and a gate voltage v.sub.ge which is given through a gate resistor 6 is biased to the positive side to turn on the IGBT 7. If the switching command v.sub.in is negative, the transistor 3 is placed in the off state, the transistor 4 is placed in the on state, the gate voltage v.sub.g becomes a negative voltage and the gate voltage is biased to the negative side to turn off the IGBT 7.
A gate power supply for a forward bias (voltage E.sub.p) 1 and a gate power supply for a negative bias (voltage E.sub.n) 2 are respectively connected to the transistors 3, 4 through a limiting resistor 5. In this case, the limiting resistor 5 is connected between the gate power supply 2 and the transistor 4 or to both of them.
IGBT has equivalent capacitances C.sub.ge 8, C.sub.cg 9, C.sub.ce 10 and the like among the gate, emitter and collector terminals as shown in FIG. 3. For this reason, a short state arises between the collector and emitter in a high frequency condition. C.sub.cg +C.sub.ge is present as an input capacitance C.sub.ies between the gate and emitter of the IGBT. Therefore, in order to make IGBT effect a switching operation, it is required charge and discharge in the input capacitance C.sub.ies through a gate resistor 6.
In a conventional gate drive circuit shown in FIG. 1, there arises a delay in time constant (R6.multidot.C.sub.ies) which is determined by the gate resistor 6 and the input capacitance C.sub.ies, which sometimes causes a trouble in the switching operation of the IGBT.
In order to make such a delay in time constant smaller, it is considered that a value of a gate resistance is made to be smaller or a voltage En of the gate power supply 2 which gives a negative bias is made to be higher.
However, when a value of the gate resistance 6 is made to smaller, a turn-off speed of the IGBT 7 becomes larger and a surge voltage becomes higher and thus the IGBT has a risk to have a damage by an overvoltage. When a voltage En of the gate power supply 2 is biased higher, a similar problem happens as well.
A current drive switching element and a voltage switching element will be considered as a switching element used in a power converter.
A turn-off characteristic of a current drive switching element, such as GTO in general is that when a load current is smaller, a turn-off time is shorter but when the load current is larger, the turn-off time is longer due to an influence of an accumulated charge in a semiconductor element as shown in FIG. 4. Therefore, in a power converter such as an inverter in which a current drive switching element is used, an on-gate supply inhibiting time (which is called as a dead time) to respective semiconductor elements of the positive and negative arms is relatively taken long and longer than the maximum turn-off time in consideration of the maximum turn-off time when the maximum current of the element is turned off and thereby short-circuit between the positive and negative arms (direct current short) is prevented.
On the other hand, it has actually observed that a turn-off characteristic of a voltage drive switching element such as an insulted gate switching element is an absolutely inverse characteristic to that of a current drive semiconductor switching element as shown in FIG. 5, that is a characteristic when a load current is larger, a turn-off time is shorter, but when the load current is smaller, the turn-off time is longer.
The reason why is that, as shown in FIG. 6, since a capacitance of a gate is larger when a voltage between a collector and emitter is smaller (such as in the on state of the device), but a capacitance of a gate is smaller (largely changed with two orders in magnitude) when a voltage between a collector and emitter is larger, charge to a gate capacitance from the collector side is slowed if a load current is very small.
For this reason, in an insulated gate switching element, it is only required in order to prevent short-circuit (direct current short-circuit) between the positive and negative arms that a dead time for the positive and negative arms is set longer in consideration of a turn-off time when a very small current of the element is turned off. However, in that case, a feature of an insulated gate switching element having a high speed switching characteristic cannot be utilized. While a higher switching frequency is desired in order to make a load current assume a sine wave form, perfect as much as possible in the case of PWM inverter and the like, the upper limit of frequency is restricted due to the restriction on the dead time.